The objective of the proposed research is to elucidate the role of circulating testosterone in behavioral changes that accompany the aging process. The above question is examined in two experiments by studying three testosterone-dependent behaviors (copulatory, aggressive, and maternal behavior) in aging male mice of two widely different genotypes (DBA and C57BL) each selected for relative physical and behavioral robustness from a larger pool of animals. In the first experiment, testosterone-dependent behavior and plasma testosterone concentrations from adulthood to old age are measured in a cross-sectional study in order to examine possible correlations between these two variables in healthy aged male mice. In the second experiment, the role of circulating testosterone in the behavioral deficits of aged animals is further examined by recording behavior and circulating testosterone in healthy aged animals subcutaneously implanted with testosterone-filled, controlled-release, silastic capsules. The animals in both experiments will be continuously monitored for blood testosterone levels by utilizing recently developed sequential bleeding techniques and microradioimmunoassay. The above procedures will provide a distinct advantage over most previous studies in which testosterone was administered by injection and single blood samples were collected. The results should substantially contribute to our knowledge of the role of circulating testosterone in behavioral changes that accompany aging in healthy organisms. This ultimately may be important for the treatment of clinical disorders in the human elderly.